FIG. 1 shows a half duplex transponder utilizing two frequencies, as is known in the prior art, generally as 100. The transponder comprises a parallel resonant (tank) circuit consisting of inductor LR capacitor CR. A capacitor CL is coupled between the parallel resonant circuit and ground. A diode D1 is coupled across the tank circuit and a capacitor CL. One or more trimming capacitors CT1 . . . CTx are coupled across the diode D1. A modulation capacitor CM is connected across the diode D1 via transistor 102 which has its drain connected to ground.
Trimming capacitors CT1 through CTx may be placed in the circuit in order to trim the oscillating frequency of the resonant circuit to 134.2 kHz, for example. The transponder is passive and it is powered by the reader sending a RF signal, which is then rectified and stored as capacitor CL and used to operate the transponder. The transponder will acknowledge receipt of a query signal, utilizing the 134.2 kHz frequency. This frequency will also be used to modulate the transistor 102 to generate one state of a digital signal, for example, a digital zero. In order to generate a signal representative of a digital one, transistor 102 is turned on, thereby coupling modulation capacitor CM across the resonant tank circuit and changing the resonant frequency to 124.2 kHz, for example. The reader circuit, which transmits the initial signal, can then interpret the difference in frequency, here 10 kHz, to obtain the information transmitted by the transponder.